Conventionally, designs of communication networks have been limited to one of two approaches. In a first approach, communication protocols for use in mesh networks configure both portable nodes as well as with static nodes to communicate using omni-directional antennas. One example of a mesh protocol is the USAP protocol. FIG. 1 is a conventional communications network wherein all nodes are configured to communicate using only omni-directional antennas. FIG. 1 shows static node Z, and portable nodes A-E. Each link to static node Z from a portable node occupies two signals (e.g. transmit and receive). Thus, a mesh system with only omni-directional antennas requires ten orthogonal signals.
Unfortunately, this approach limits the capacity of mobile networks because, for example, practical communications options depend on orthogonal links (see Cover, Thomas M. and Joy A. Thomas. Elements of Information Theory. New York: John Wiley & Sons, 1991; Vanhaverbeke, F. and M. Moeneclaey. “Sum Capacity of Equal-Power Users in Overloaded Channels.” IEEE Transactions Communications 53.2 (February 2005): pp. 228-33). Such a requirement requires a tradeoff between time and bandwidth. For example, keeping bandwidth constant, for example, in a system where all signals use the same frequency, may require extra time for transmit and receive because the system may cycle through all of the nodes until the appropriate node is found. On the other hand, sending out all of the signals at the same time requires additional bandwidth. Thus, increased capacity in a wholly omni-directional system disadvantageously requires either more time or more bandwidth.
In a second approach, directional antennas may be used to increase the capacity of the system via frequency reuse. FIG. 2 is a conventional communications network with directional antennas disposed at a static node. In FIG. 2, static node Z is equipped with directional antennas, and portable nodes A-E are disposed within the paths of the directional antennas. A network of nodes according to this second approach usually is partitioned into distinct clusters. Each cluster of nodes has a node that acts as a cluster-head. A cluster-head node conventionally controls most, if not all, communication between clusters. In most clustering techniques, a cluster is formed such that all cluster member nodes are within 1 hop of the cluster-head. Therefore access to the cluster-head is critical for efficient network operation.
This arrangement theoretically allows for complete frequency reuse. Thus, static node Z can communicate with five portable nodes A-E over the same frequency at the same time, resulting in a fivefold increase in capacity. When portable nodes A-E are transmitters and static node Z is a receiver, less power is required at the transmitters, which implies, for example, that there is less interference into adjacent regions. With little noise between layers comes the opportunity to reuse bandwidth. Alternatively, this arrangement requires less hops to the destination at the same power level. The mobile fading envelope is less severe, and this reduces contention among signals to Z that can cause bottlenecks. There also may be less multipath (see Durgin, G. D., V. Kukshya, and T. S. Rappaport. “Wideband measurements of angle and delay dispersion for outdoor and indoor peer-to-peer radio channels at 1920 MHz.” IEEE Transactions on Antennas and Propogation 51.5 (May 2003): 936-944; Petrus, P., J. H. Reed, and T. S. Rappaport. “Effects of directional antennas at the base station on the Doppler spectrum.” IEEE Communications Letters 1.2 (March 1997): 40-42). Thus, cluster-head convergence results in a system that is quicker under bias. If a given topology is arranged well, a given node is more likely to be in a cluster, so a signal need not go through a mesh to get to a cluster head.
Unfortunately, this second approach also has its disadvantages. For example, it often is not practical to equip all nodes with directional antennas. It also is too restrictive to assume that all nodes will be within the range of directional antennas. Also, problems may arise when multiple nodes are within the path of a single directional antenna. For example, FIG. 3 is an exemplary communications network where two nodes are observable by a single directional antenna. In FIG. 3, portable nodes X and Y are both within the path of the directional antenna of static node Z. Both X and Y may receive and process the message from static node Z. This may result in extraneous messages being relayed through the network, for example, using the limited resources inappropriately.
Thus, it will be appreciated that there is a need in the art for a system and/or method to overcome one or more of these and/or other disadvantages.